LED switching power supply PCB board design seven steps
In the design of switching power supply, if the PCB board is not properly designed, it will radiate too much electromagnetic interference. The PCB board design with stable power supply is now summarizing the seven-step tricks: through the analysis of the matters that need to be paid attention to in each step, the PCB board design can be easily done step by step!
The design process from schematic diagram to PCB establishes component parameters -> input principle netlist -> design parameter setting -> manual layout -> manual wiring -> verify design -> review -> CAM output.
Parameter setting The spacing between adjacent wires must meet the electrical safety requirements, and for the convenience of operation and production, the spacing should be as wide as possible. The minimum spacing must be at least suitable for the withstand voltage. When the wiring density is low, the spacing of the signal lines can be appropriately increased. For signal lines with high and low levels, the spacing should be as short as possible and the spacing should be increased. Set the trace spacing to 8mil. The distance from the edge of the inner hole of the pad to the edge of the printed board should be greater than 1mm, which can avoid pad defects during processing. When the traces connected to the pads are thin, the connection between the pads and the traces should be designed in a drop shape. The advantage of this is that the pads are not easy to peel, but the traces and the pads are not easy to disconnect.
The practice of component layout has proved that even if the circuit schematic is designed correctly, the printed circuit board is not properly designed, which will have an adverse effect on the reliability of electronic equipment. For example, if the two thin parallel lines of the printed board are close together, the delay of the signal waveform will be formed, and reflection noise will be formed at the end of the transmission line; the interference caused by the inconsiderate consideration of the power supply and ground wire will make the product's Therefore, when designing a printed circuit board, care should be taken to adopt the correct method.
Each switching power supply has four current loops: (1), power switch AC loop (2), output rectification AC loop (3), input signal source current loop (4), output load current loop
The input circuit charges the input capacitor through an approximately DC current, and the filter capacitor mainly acts as a broadband energy storage; similarly, the output filter capacitor is also used to store high-frequency energy from the output rectifier, while eliminating the DC energy of the output load circuit . Therefore, the terminals of the input and output filter capacitors are very important, and the input and output current loops should only be connected to the power supply from the terminals of the filter capacitors; if the connection between the input/output circuit and the power switch/rectifier circuit cannot be connected with the capacitor The terminals are directly connected, and the AC energy will be radiated to the environment by the input or output filter capacitor. The AC loop of the power switch and the AC loop of the rectifier contain high-amplitude trapezoidal currents. The harmonic content of these currents is very high. The frequency is much higher than the switching fundamental frequency. about 50ns. These two circuits are most likely to generate electromagnetic interference, so these AC circuits must be laid out before other printed lines in the power supply are wired. The three main components of each circuit, filter capacitors, power switches or rectifiers, inductors or transformers, should be connected to each other. Place them adjacent to each other, adjusting the position of the components so that the current path between them is as short as possible.
The best way to build a switching power supply layout is similar to its electrical design, and the best design flow is as follows:
Place the transformer Design the power switch current loop Design the output rectifier current loop connected to the control circuit of the AC power supply circuit Design the input current source loop and input filter Design the output load loop and output filter According to the functional unit of the circuit, carry out all the components of the circuit When laying out, the following principles must be followed:
(1) The PCB size should be considered first. When the PCB size is too large, the printed lines will be long, the impedance will increase, the anti-noise ability will decrease, and the cost will also increase; if the size is too small, the heat dissipation will not be good, and adjacent lines will be easily disturbed. The best shape of the circuit board is rectangular, with an aspect ratio of 3:2 or 4:3. The components located on the edge of the circuit board are generally not less than 2mm from the edge of the circuit board.
(2) When placing the device, consider the future welding, not too dense;
(3) Take the core component of each functional circuit as the center and make layout around it. Components should be evenly, neatly, and compactly arranged on the PCB, minimize and shorten the leads and connections between components, and decoupling capacitors should be as close as possible to the VCC of the device.
(4) For circuits operating at high frequencies, the distribution parameters between components must be considered. Generally, the circuit should arrange the components in parallel as much as possible. In this way, it is not only beautiful, but also easy to assemble and weld, and easy to mass produce.
(5) Arrange the position of each functional circuit unit according to the flow of the circuit, so that the layout is convenient for signal circulation, and the direction of the signal is kept as consistent as possible.
(6) The first principle of layout is to ensure the pass rate of wiring, pay attention to the connection of flying wires when moving devices, and put devices with connection relationship together.
(7) Reduce the loop area as much as possible to suppress the radiation interference of the switching power supply.
the fourth step
Wiring Switching power supplies contain high-frequency signals, and any printed line on the PCB can act as an antenna. The length and width of the printed line will affect its impedance and inductive reactance, thereby affecting the frequency response. Even traces passing DC signals can couple to RF signals from adjacent traces and cause circuit problems (or even radiate unwanted signals again). Therefore, all printed lines passing AC current should be designed as short and wide as possible, which means that all components connected to printed lines and connected to other power lines must be placed very close. The length of a trace is directly proportional to the inductance and impedance it exhibits, while the width is inversely proportional to the inductance and impedance of the trace. The length reflects the wavelength at which the printed line responds. The longer the length, the lower the frequency at which the printed line can send and receive electromagnetic waves, and the more radio frequency energy it can radiate. According to the size of the current of the printed circuit board, try to increase the width of the power line to reduce the loop resistance. At the same time, make the direction of the power line and ground line consistent with the direction of the current, which helps to enhance the ability to resist noise. Grounding is the bottom branch of the four current loops of the switching power supply. It plays a very important role as the common reference point of the circuit. It is an important method to control interference. Therefore, the placement of ground wires should be carefully considered in the layout, and mixing various grounds will cause unstable operation of the power supply.
The following points should be paid attention to in the ground wire design:
1. Correct selection of single-point grounding Usually, the common terminal of the filter capacitor should be the only connection point where other grounding points are coupled to the high-current AC ground. It should be connected to the grounding point of this level, mainly because the current flowing back to the ground from each part of the circuit changes, and the impedance of the actual flowing line will cause the ground potential of each part of the circuit to change and introduce interference. In this switching power supply, its wiring and the inductance between devices have little influence, while the circulating current formed by the grounding circuit has a greater influence on interference, so a one-point grounding is adopted, that is, the grounding wires of several devices in the switching power supply current loop Connect to the ground pin, and the ground wires of several devices that output the current loop of the rectifier are also connected to the ground pin of the corresponding filter capacitor, so that the power supply works more stably and is not easy to self-excite. When a single point cannot be achieved, the common ground Connect two diodes or a small resistor, in fact, it can be connected to a relatively concentrated piece of copper foil.
2. Try to make the grounding wire as thick as possible. If the grounding wire is very thin, the grounding potential will change with the change of current, which will cause the timing signal level of the electronic equipment to be unstable and the anti-noise performance to deteriorate. Therefore, it is necessary to ensure that the grounding terminal of each large current Use printed lines as short and wide as possible, widen the width of the power supply and ground wires as much as possible, preferably the ground wire is wider than the power line, their relationship is: ground wire>power wire>signal wire, if possible, the ground wire The width should be greater than 3mm, and a large area of copper layer can also be used as a ground wire, and the unused places on the printed board are connected to the ground as the ground wire. When performing global routing, the following principles must also be followed:
(1) Wiring direction: From the perspective of the welding surface, the arrangement of the components should be as consistent as possible with the schematic diagram, and the wiring direction should preferably be consistent with the wiring direction of the circuit diagram, because various parameters are usually required on the welding surface during the production process Therefore, this is convenient for inspection, debugging and maintenance in production (Note: refers to the premise of meeting the requirements of circuit performance, complete machine installation and panel layout).
(2) When designing the wiring diagram, the wiring should have as few turns as possible, and the line width on the printing arc should not change suddenly. The corner of the wire should be ≥ 90 degrees, and the lines should be simple and clear.
(3) Cross circuits are not allowed in the printed circuit. For lines that may cross, two methods of "drilling" and "winding" can be used to solve them. That is to let a lead wire "drill" through the gap under the feet of other resistors, capacitors, and triodes, or "wrap" through one end of a lead wire that may cross. In special cases, even if the circuit is very complicated, it is also allowed to simplify the design Use wire jumpers to solve cross circuit problems. Due to the use of a single-sided board, the in-line components are located on the top side, and the surface-mount devices are located on the bottom side, so the in-line devices can overlap with the surface-mount devices during layout, but overlapping of pads should be avoided. 3. Input ground and output ground This switching power supply is a low-voltage DC-DC. If you want to feed the output voltage back to the primary of the transformer, the circuits on both sides should have a common reference ground. Therefore, after laying copper on the ground wires on both sides, They must also be connected together to form a common ground.
the fifth step
Inspection: After the wiring design is completed, it is necessary to carefully check whether the wiring design complies with the rules set by the designer. At the same time, it is also necessary to confirm whether the established rules meet the requirements of the printed board production process. Generally, check lines and lines, lines and component pads , Whether the distance between the line and the through hole, the component pad and the through hole, the through hole and the through hole is reasonable, and whether it meets the production requirements. Whether the width of the power line and the ground line is appropriate, and whether there is a place in the PCB where the ground line can be widened. Note: Some errors can be ignored. For example, part of the Outline of some connectors is placed outside the board frame, and errors will occur when checking the spacing; in addition, copper must be poured again every time the traces and vias are modified.
The review is based on the "PCB Checklist", which includes design rules, layer definitions, line widths, spacing, pads, and via settings. It is also important to review the rationality of device layout, routing of power and ground networks, and high-speed clock networks. The wiring and shielding, the placement and connection of decoupling capacitors, etc.
a. The layers that need to be output include wiring layer (bottom layer), silk screen layer (including top layer silk screen, bottom layer silk screen), solder mask layer (bottom layer solder mask), drilling layer (bottom layer), and drill file (NCDrill)
b. When setting the Layer of the silk screen layer, do not select PartType, select the top layer (bottom layer) and the Outline, Text, and Line of the silk screen layer
c. When setting the Layer of each layer, select Board Outline. When setting the Layer of the silk screen layer, do not select PartType, and select the Outline, Text, and Line of the top layer (bottom layer) and the silk screen layer.
d. When generating the drilling file, use the default settings of PowerPCB and do not make any changes.